Optoelectronic packet switch/routers

ABSTRACT

A packet switch/router including a first stage switch fabric receiving an electrical signal, a mid-stage buffer receiving and storing the electrical signal from the first stage switch fabric, and a second stage switch fabric receiving the electrical signal from the mid-stage buffer. Each switch fabric includes N layers of N×N arrayed waveguide gratings (AWGs), and each AWG has ingress ports and egress ports. A wavelength tunable device, such as a tunable laser, communicates with a source ingress port of an AWG and converts the received electrical signal to an optical signal having a wavelength selected for routing a packet from the source ingress port to a target egress port of the arrayed waveguide grating. A photoreceiver, such as a burst-mode photoreceiver, receives the propagated optical signal from the target egress port and converts the optical signal to the electrical signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This U.S. patent application is a continuation of and claims priorityunder 35 U.S.C. §120 from, U.S. patent application Ser. No. 13/552,774,filed on Jul. 19, 2012, which is hereby incorporated by reference in itsentirety.

TECHNICAL FIELD

This disclosure relates to optoelectronic packet switch/routers usingmulti-layer multi-stage array waveguide gratings.

BACKGROUND

The volume of data handled by network devices has increased dramaticallyover the past decade, resulting in significant challenges in terms ofdesigning bigger and faster switches/routers to keep up with increasingbandwidth demands. The total off-chip bandwidth of the switching/routingchip C is the product of the total number of ports per chip R (referredto as the radix of the chip) and the per port speed B (C=R×B) Increasingthe total off-chip bandwidth C requires increasing the radix of the chipR and/or the per port speed B. Flowever, there is a trade-off betweenthese two parameters in terms of electronic integrated circuit (IC)design.

It is difficult to increase the total off-chip bandwidth C of an ICswitching/routing chip. First, the length of on-chip interconnectsgenerally governs the speed between ports. The smaller the size of thechip, the faster the speed the interconnects can support. Therefore, asmaller chip may be able to support higher port speeds, but only at thecost of lower port count due to the smaller chip area (and hence limitedpin count; of which power supplies typically consume almost half of thepins). Second, the input/output (I/O) port power density typically doesnot scale with the transistor technology. Even if the transistortechnology supports higher speed and lower power, the I/O powerincreases as the port speed increases. More effective on-chipequalization components are needed to drive signals across a reasonablelength of the electrical transmission channels at higher speeds, each ofwhich produces additional heat. Since there is a maximum heatdissipation capacity for a given die size, the port count is reduced tofit into the power envelope for increased per port speed. To avoid theseissues and achieve higher switching capacity per node, one commonpractice is to connect multiple chips in a non-blocking fashion inside achassis to form a node with a relatively larger aggregate switchingcapacity. However, this approach has its own limitations, as thedistance over the chassis backplane's Printed Circuit Board (PCB) that aswitch chip I/O can drive signals becomes shorter as the I/O rateincreases. Another general practice is to interconnect multiple smallchassis together in a non-blocking fashion to form a virtual chassiswith larger switching capacity. This approach is the least integrated,hence the most expensive and power hungry because it represents asignificant overhead in terms of packaging materials and interconnectsbetween chassis, and also poses many challenges in terms of deploymentand operations.

Arrayed waveguide gratings (AWGs) have been used as interconnectionfabrics in all-optical routers and optical-electrical hybrid routers. Aknown implementation includes single AWG devices or two cascaded AWGswith optical label switching, all optical wavelength converter(s) andshared optical delay line buffers. This system is limited by the size ofthe AWG and is difficult to scale. In addition optical delay linebuffers, which are typically very small, are generally insufficient tomake the router of practical use. The two-stage cascaded design makesuse of one AWG to solve packet contention rather than scaling up theport count. Other known implementations include four layers of 4×4 AWGsand shared CMOS RAM to provide a single stage design with multiplelayers of AWGs; however, this design does not result in a non-blockingfabric, making the design inefficient and unappealing from a practicaldeployment perspective. This approach is also very difficult to scaledue to the complexity and limitations of the shared RAM architecture.

SUMMARY

A hybrid optoelectronic switch/router architecture may overcome thetotal switching capacity barrier in electronic integrated circuits,while providing a relatively large capacity using commercially availablecomponents. One aspect of the disclosure provides a packet switch/routerthat includes a first stage switch fabric receiving an electricalsignal, a mid-stage buffer receiving and storing the electrical signalfrom the first stage switch fabric, and a second stage switch fabricreceiving the electrical signal from the mid-stage buffer. Each switchfabric includes N layers of N×N arrayed waveguide gratings (ANA/Gs), andeach AWG has ingress ports and egress ports. A wavelength tunabledevice, such as a tunable laser, communicates with a source ingress portof an AWG and converts the received electrical signal to an opticalsignal having a wavelength selected for routing a packet from the sourceingress port to a target egress port of the arrayed waveguide grating. Aphotoreceiver, such as a burst-mode photoreceiver, receives thepropagated optical signal from the target egress port and converts theoptical signal to the electrical signal.

Implementations of the disclosure may include one or more of thefollowing features. In some implementations, the packet switch/routerincludes an interface, such as an optical transceiver, receiving anoptical signal and converting the optical signal to the electricalsignal received by the first stage switch fabric. The packetswitch/router may include an input buffer receiving and storing theelectrical signal from the interface. The input buffer communicates theelectrical signal to the first stage switch fabric. Each switch stagefabric may be non-blocking and/or connected in a Clos network. In someexamples, each switch stage fabric may include 100 layers of 100×100arrayed waveguide gratings having a per port speed of about 10G, whichwork together with fast wavelength converters to steer the packetsthrough the fabric. Such a system may achieve a total switching capacityof ˜100 Tbps.

In some implementations, the packet switch/router includes a schedulerin communication with the interface, the input buffer, the mid-stagebuffer, and wavelength tunable devices. The scheduler receivesnotification from the interface of a received packet on the electronicsignal and issues steering instructions to the wavelength tunabledevices for routing the packet from a source ingress port to a targetegress port on each switch fabric. The scheduler may issue instructionsin parallel to the first and second stage switch fabrics. The mid-stagebuffer reduces scheduling speed requirements for the wavelength tunabledevices by a factor of N and allows the scheduler to send routinginstructions in parallel to the first and second switch fabrics.

Fast tunable lasers running at 10 Gbps as wavelength converters alongwith 10 Gbps burst-mode receivers and electronic buffers are practicaland commercial components that can meet the technical specifications ofthe proposed architecture. In addition, by implementing mid-stagebuffers, the scheduling speed requirement on the scheduler can bereduced by N times.

Another aspect of the disclosure provides a method of packet switching.The method includes receiving an electric signal, converting theelectric signal to a first optical signal, and propagating the firstoptical signal through a first stage switch fabric. The first stageswitch fabric includes N layers of N×N arrayed waveguide gratings, whereeach arrayed waveguide grating has ingress ports and egress ports. Thefirst optical signal has a first wavelength that results in routing apacket from a source ingress port to a target egress port of an arrayedwaveguide grating of the first stage switch fabric. The method alsoincludes converting the first optical signal back to the electricsignal, converting the electric signal to a second optical signal, andpropagating the second optical signal through a second stage switchfabric. The second stage switch fabric includes N layers of N×N arrayedwaveguide gratings, where each arrayed waveguide grating has ingressports and egress ports. The second optical signal has a secondwavelength that results in routing the packet from a source ingress portto a target egress port of an arrayed waveguide grating of the secondstage switch fabric. The method includes converting the second opticalsignal back to the electric signal.

In some implementations, the method includes receiving an input opticalsignal and converting the input optical signal to the electrical signal.The method may include storing the electrical signal while waiting forinstructions fir converting the electrical signal to the first opticalsignal having the first wavelength. Moreover, the method may includereceiving routing instructions for converting the electrical signal tothe first optical signal. After converting the first optical signal backto the electric signal, the method may include storing the electricalsignal while waiting for instructions for converting the electricalsignal to the second optical signal having the second wavelength. Themethod includes receiving routing instructions for converting theelectrical signal to the second optical signal. In some examples, themethod includes receiving in parallel routing instructions forconverting the electrical signal to the first optical signal and routinginstructions for converting the electrical signal to the second opticalsignal. The method may include converting the second optical signal backto the electric signal using a burst-mode photoreceiver.

The method may include connecting the first and second stage switchfabrics in a Clos network. Each switch fabric may include 100 layers of100×100 arrayed waveguide gratings having a per port speed of about 10G.

The details of one or more implementations of the disclosure are setforth in the accompanying drawings and the description below. Otheraspects, features, and advantages will be apparent from the descriptionand drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of an exemplary packet switch/router.

FIG. 2 is a schematic view of an exemplary arrayed waveguide grating.

FIG. 3 is a schematic view of an exemplary a multi-stage packetswitch/router.

FIG. 4 is a schematic view of an exemplary Clos network.

FIG. 5 is a schematic view of an exemplary packet switch/router.

FIG. 6 is a schematic view of an exemplary arrangement of operations fora method of packet switching.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIGS. 1-3, in some implementations, a packet switch/router100 has a hybrid optical and electrical architecture that can support arelatively large capacity, such as 100 Tbps. The packet switch/router100 may include a first stage switch fabric 120 receiving an electricalsignal 10, a mid-stage buffer 130 receiving and storing the electricalsignal 10 from the first stage switch fabric 120, and a second stageswitch fabric 140 receiving the electrical signal 10 from the mid-stagebuffer 130. The packet switch/router 100 may include input buffer 110for receiving and storing the electric signal 10 before delivery to thefirst stage switch fabric 120. Although only two switch fabric stages120, 140 are shown, the packet switch/router 100 may include more thantwo switch fabric stages 120, 140, each with a mid-stage butter 130 incommunication therebetween.

The packet switch/router 100 may use a combination of N×N arrayedwaveguide gratings (AWGs) 200. By placing 2N layers of N×N AWGs 200 inthe first and second stages 120, 140 one can achieve N² ports. An AWG200 is a passive planar light wave circuit device typically used inoptical networks as a wavelength multiplexer and/or demultiplexer. N×NAWGs 200 also have wavelength routing capabilities, as shown in FIG. 2.If a system has N equally-spaced wavelengths λ_(N), an N×N AWG 200 canbe designed with an egress port spacing matching the wavelength spacing.The N×N AWG 200 routes differing wavelengths at an ingress port 210 todifferent egress ports 220 such that all N wavelengths are mapped to allN egress ports 220 _(N) sequentially. The routing of the same Nwavelengths at two consecutive ingress ports 210 have the wavelengthmapping shifted by one egress side, as shown in FIG. 2, which is notedas cyclic. Titus, the N×N AWG 200 can route any incoming packet 20 todesired egress ports 220 by controlling their wavelengths at the ingressport(s) 210 through a wavelength tunable device 230, such as a tunablelaser (e.g., running at 10 Gbps). The wavelength tunable device, 230communicates with a source ingress port 210 s of an AWG 200 and convertsthe received electrical signal 10 to an optical signal 10 having awavelength λ selected for routing a packet 20 from the source ingressport 210 s to a target egress port 220 t of the AWG 200. A photoreceiver240, such as a burst-mode photoreceiver, receives the propagated opticalsignal 10 from the target egress port 220 t and converts the opticalsignal 10 to the electrical signal 10.

Using a symmetric N×N AWG 200 alone may pose a limitation on the totalswitching capacity. Per port speed is limited by tunable lasers 230 atthe ingress ports 210 and burst-mode photoreceivers 240 at the egressports 220. Moreover, it may only be practical to find cost-effectivecomponents with a relatively fast response time ns to be able to processeach packet 20) at 10 Gbit speeds. An AWG 200 can be fabricated as a100×100-port device with 50 GHz spacing. Currently, fabricating an N×NAWG with N>100 with concomitantly narrower wavelength spacing is verychallenging due to the diffraction limit of the grating design. As aresult, the current maximum switching capacity of a single AWG is about1 Tbps, which is comparable to a commercial Silicon IC chip at thepresent moment.

Referring to FIG. 3, in some implementations, the packet switch/router100 may include multiple stages 120, 140 with N AWGs 200 in each stage120, 140. The multi-stage architecture increases a switching capacity bya factor of N. In the example shown, the packet switch/router 100includes first and second stages 120, 140; however, many stages arepossible. The first and second stages 120, 140 each include N layers ofN×N AWGs 200. Although shown in a perpendicular (vertical andhorizontal) arrangement, any orientation is possible, as needed for thefabrication. The egress ports 220 of the N layers of N×N AWGs 200 of thefirst stage 120 communicate with (e.g., connect to) the ingress ports210 of the N layers of N×N AWGs 200 of the second stage 140, where eachstage has exactly N² ingress ports 210 and N² egress ports 220.

Referring to FIG. 4, in some implementations, the stages 120, 140 areconnected in a Clos network 400 (e.g., a two-stage Clos network for twostages). Each ingress port 210 of each AWG 200 may have a wavelengthcontrol device 230, such as a tunable laser or a wavelength converter,and packet buffers 110, 130. This configuration generates a non-blockingN²×N² switch fabric 120, 140 because any packet 20 at any one of the N²ingress port can be routed through the first-stage AWG 200 to a desiredsecond stage AWG 200, and then routed through the second-stage AWG 200to a desired egress port 220. If the per port speed is B, then the totalcapacity becomes B×N² with the use of 2N AWGs 200, which compares to acapacity of B×N for one AWG 200.

Referring to FIG. 5, in some implementations, the packet switch/router100 includes a control plane 510 and a data plane 520, providing aseparation between managing packet routes and routing the packets 20.The control plane 510 may operate in the electrical domain. In someexamples, the control plane 510 includes a scheduler 512 (e.g., an ASIC,FPGA or NPU) to schedule packet routing and configure the wavelengthsλ_(N) at each port 210, 220. The data plane 520 may operate at leastprimarily in the optical domain using the multi-stage non-blocking N×NAWG based optical switching fabric 120, 140. The data plane 520 mayinclude electronic buffers 110, 130 (e.g., when well sized opticalbuffers 110, 130 are not practical or realizable).

The relatively small buffer solution may perform well when trafficarriving at the switch 100 is 1) well aggregated and 2) multiplexed witha relatively very low level of burstiness, a set of preconditions whichare frequently not realized in the real world, the former being acharacteristic of a network's topology and demand and the latter beingdifficult to achieve without very sophisticated flow/admission control.

The scheduler 512 may communicate with an I/O interface 150, the inputbuffer 110, the mid-stage buffer 130, and the wavelength tunable devices230. The scheduler 510 receives notification from the interface 150 of areceived packet 20 on the electronic signal and issues steeringinstructions to the wavelength tunable devices 230 for routing thepacket 20 from a source ingress port 210 s to a target egress port 220 ton each switch fabric 120, 140. The scheduler 512 may issue instructionsin parallel to the first and second stage switch fabrics 120, 140.

The data plane 520 can be separated into the following four logicalblocks architecturally: (a) system I/O interface 150 and input buffer110, (b) first-stage fabric 120, (c) mid-stage buffer 130, and (d)second-stage fabric 140. The system I/O interface 150 and the electricalbuffer 110 interconnect the packet switch/router 100 with otherswitches, servers or transponders. The front panel I/O interfaces 150can be optical transceiver modules that convert optical signals 10 e toelectrical signals 10 e and then send the signals 10 e to both thecontrol plane 510 and the data plane 520. The control plane 510 takesthe signal 10 e and examines a packet header to determine how to route acorresponding packet 20 through the fabric to a target egress port 220,220 t by setting up the wavelength tunable devices 230—fast tunablelasers—at the ingress port 210 of each AWG 200.

The input buffer 110 first receives the signal 10 e to the data plane520 while waiting for the control plane 510 to determine how to steerthe signal 10, 10 e, 10 o through the first-stage fabric 120, whichincludes N layers of MN AWGs 200. After receiving control planeinstructions/results, the input buffer 110 sends the packet 20 to thewavelength tunable device 230 at the ingress port 210, 210 s, whichconverts the electrical signal 10 e to a first optical signal 10 o ₁with a first wavelength λ₁ based on the information from the controlplane 510 for routing the packet 20 to a target egress port 220, 220 tof the first stage 120. A photoreceiver 240 receives the packet 20 fromthe first stage egress port 220 and converts the first optical signal 10o ₁ back to an electrical signal 10 e.

The mid-stage buffer 130 stores the electrical signal 10 e before it issent to the second switch fabric stage 140. The mid-stage buffers 130reduce the scheduling speed requirement on the control plane 510 by afactor of N (e.g., by a factor of 100 for 100×100 AWGs). The schedulingspeed is determined by data rate/packet size×header size perpacket×number of ports (e.g., scheduling speed=(10 Gbps)/(1000Bytes)×(10 Bytes)×number of ports). The mid-stage buffer 130 allows thescheduler 512 to work on one switch fabric stage 120, 140 instead of allswitch fabric stages 120, 140 at the same time. As a result, thescheduler 512 only needs to handle N ports 210 with mid-stage buffers130 versus N² ports without the mid-stage buffers 130, reducing thecontrol plane speed requirement by a factor of N. The mid-stage buffer130 may receive instructions from the scheduler 512 for converting thestored electrical signal 10 e to a second optical signal 10 o ₂ having asecond wavelength λ₂ using the wavelength tunable device 230, such as atunable laser.

The AWGs 200 of the second switch fabric stage 140 route the packets 20to their final egress ports 220 by setting up the second stagewavelength tunable devices 230 attached to the ingress ports 210 secondstage AWGs 200 correct wavelengths λ in response to control planeresults. The optical signal 10 o at the egress ports 220 of the secondswitch fabric stage 140 then gets fed back to the system I/O interface150 through optical fibers 160 and photoreceivers 240. When thewavelength tunable device 230 (e.g., fast tunable laser) switches fromone wavelength λ to another, there may be no signal 10 during theswitching period before the new wavelength λ gets stabilized. Althoughthe switching time is very fast and likely approximately Ins, this mayrequire burst-mode photo-receivers 240 (e.g., operating at 10 Gbps) atthe egress ports 220 of each AWG 200. Since this is an input-bufferedsystem, existing scheduling algorithms published for input-bufferedsystems with virtual output queues (VOQs) can be used.

FIG. 6 provides an exemplary arrangement of operations for a method ofpacket switching. The method includes receiving 602 an electric signal10, converting 604 the electric signal 10, 10 e to a first opticalsignal 10 o ₁, and propagating 606 the first optical signal 10 o ₁through a first stage switch fabric 120. The first stage switch fabric120 includes N layers of N×N arrayed waveguide gratings (AWGs) 200,where each AWG 200 has ingress ports 210 and egress ports 220. The firstoptical signal 10 o ₁ has a first wavelength λ₁ that results in routinga packet 20 from a source ingress port 210 s to a target egress port 220t of an AWG 200 of the first stage switch fabric 120. The method alsoincludes converting 608 the first optical signal 10 o ₁ back to theelectric signal 10 e, converting 610 the electric signal 10 e to asecond optical signal 10 o ₂, and propagating 612 the second opticalsignal 10 o ₂ through a second stage switch fabric 140. The second stageswitch fabric 140 includes N layers of N×N AWGs 200, where each AWG 200has ingress ports 210 and egress ports 220. The second optical signal 10o ₂ has a second wavelength λ₂ that results in routing the packet 20from a source ingress port 210 s to a target egress port 220 t of an AWG200 of the second stage switch fabric 140. The method includesconverting 614 the second optical signal 10 o ₂ back to the electricsignal 10 e.

In some implementations, the method includes receiving an input opticalsignal 10 and converting the input optical signal 10 to the electricalsignal 10 e. The method may include storing the electrical signal 10 e(e.g., in the input buffer 110) while waiting for instructions (e.g.,from the scheduler 512) for converting the electrical signal 10 e to thefirst optical signal 10 o ₁ having the first wavelength λ₁. Moreover,the method may include receiving routing instructions (e.g., from thescheduler 512) for converting the electrical signal 10 e to the firstoptical signal 10 o ₁. After converting the first optical signal 10 oback to the electric signal 10 e, the method may include storing theelectrical signal 10 e (e.g., in the mid-stage buffer 130) while waitingfor instructions (e.g., from the scheduler 512) for converting theelectrical signal 10 e to the second optical signal 10 o ₂ having thesecond wavelength λ₂ (e.g., using a wavelength tunable device 230, suchas a tunable laser). The method includes receiving routing instructions(e.g., from the scheduler 512) for converting the electrical signal 10 eto the second optical signal 10 o ₂. In some examples, the methodincludes receiving in parallel (e.g., from the scheduler 512) routinginstructions for converting the electrical signal 10 e to the firstoptical signal 10 o ₁ and routing instructions fir converting theelectrical signal 10 e to the second optical signal 10 o ₂. The methodmay include converting the second optical signal 10 o ₂ back to theelectric signal 10 e using a burst-mode photoreceiver 240.

The method may include connecting the first and second stage switchfabrics 120, 140 in a Clos network 400. Each switch fabric 120, 140 mayinclude 100 layers of 100×100 arrayed waveguide gratings 200.

Various implementations of the systems and techniques described here canbe realized in digital electronic and/or optical circuitry, integratedcircuitry, specially designed ASICs (application specific integratedcircuits), computer hardware, firmware, software, and/or combinationsthereof. These various implementations can include implementation in oneor more computer programs that are executable and/or interpretable on aprogrammable system including at least one programmable processor, whichmay be special or general purpose, coupled to receive data andinstructions from, and to transmit data and instructions to, a storagesystem, at least one input device, and at least one output device.

These computer programs (also known as programs, software, softwareapplications or code) include machine instructions for a programmableprocessor, and can be implemented in a high-level procedural and/orobject-oriented programming language, and/or in assembly/machinelanguage. As used herein, the terms “machine-readable medium” and“computer-readable medium” refer to any computer program product,non-transitory computer readable medium, apparatus and/or device (e.g.,magnetic discs, optical disks, memory, Programmable Logic Devices(PLDs)) used to provide machine instructions and/or data to aprogrammable processor, including a machine-readable medium thatreceives machine instructions as a machine-readable signal. The term“machine-readable signal” refers to any signal used to provide machineinstructions and/or data to a programmable processor.

Implementations of the subject matter and the functional operationsdescribed in this specification can be implemented in digital electroniccircuitry, or in computer software, firmware, or hardware, including thestructures disclosed in this specification and their structuralequivalents, or in combinations of one or more of them. Moreover,subject matter described in this specification can be implemented as oneor more computer program products, i.e., one or more modules of computerprogram instructions encoded on a computer readable medium for executionby, or to control the operation of, data processing apparatus. Thecomputer readable medium can be a machine-readable storage device, amachine-readable storage substrate, a memory device, a composition ofmatter effecting a machine-readable propagated signal, or a combinationof one or more of them. The terms “data processing apparatus”,“computing device” and “computing processor” encompass all apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, or multiple processors or computers.The apparatus can include, in addition to hardware, code that creates anexecution environment for the computer program in question, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them. A propagated signal is an artificially generated signal, e.g.,a machine-generated electrical, optical, or electromagnetic signal, thatis generated to encode information for transmission to suitable receiverapparatus.

A computer program (also known as an application, program, software,software application, script, or code) can be written in any form ofprogramming language, including compiled or interpreted languages, andit can be deployed in any form, including as a stand-alone program or asa module, component, subroutine, or other unit suitable for use in acomputing environment. A computer program does not necessarilycorrespond to a file in a file system. A program can be stored in aportion of file that holds other programs or data (e.g., one or morescripts stored in a markup language document), in a single filededicated to the program in question, or in multiple coordinated files(e.g., files that store one or more modules, sub programs, or portionsof code). A computer program can be deployed to be executed on onecomputer or on multiple computers that are located at one site ordistributed across multiple sites and interconnected by a communicationnetwork.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices. Moreover, a computer can be embedded inanother device, e.g., a mobile telephone, a personal digital assistant(PDA), a mobile audio player, a Global Positioning System (GPS)receiver, to name just a few. Computer readable media suitable forstoring computer program instructions and data include all forms ofnon-volatile memory, media and memory devices, including by way ofexample semiconductor memory devices, e.g., EPROM, EEPROM, and flashmemory devices; magnetic disks, e.g., internal hard disks or removabledisks; magneto optical disks; and CD ROM and DVD-ROM disks. Theprocessor and the memory can be supplemented by, or incorporated in,special purpose logic circuitry.

To provide for interaction with a user, one or more aspects of thedisclosure can be implemented on a computer having a display device,e.g., a CRT (cathode ray tube), LCD (liquid crystal display) monitor, ortouch screen for displaying information to the user and optionally akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input. In addition, a computer can interact with a user bysending documents to and receiving documents from a device that is usedby the user; for example, by sending web pages to a web browser on auser's client device in response to requests received from the webbrowser.

One or more aspects of the disclosure can be implemented in a computingsystem that includes a backend component, e.g., as a data server, orthat includes a middleware component, e.g., an application server, orthat includes a frontend component, e.g., a client computer having agraphical user interface or a Web browser through which a user caninteract with an implementation of the subject matter described in thisspecification, or any combination of one or more such backend,middleware, or frontend components. The components of the system can beinterconnected by any form or medium of digital data communication,e.g., a communication network. Examples of communication networksinclude a local area network (“LAN”) and a wide area network (“WAN”), aninter-network (e.g., the Internet), and peer-to-pee networks (e.g., adhoc peer-to-peer networks).

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someimplementations, a server transmits data (e.g., an HTML page) to aclient device (e.g., for purposes of displaying data to and receivinguser input from a user interacting with the client device). Datagenerated at the client device (e.g., a result of the user interaction)can be received from the client device at the server.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of the disclosure or of what maybe claimed, hut rather as descriptions of features specific toparticular implementations of the disclosure. Certain features that aredescribed in this specification in the context of separateimplementations can also be implemented in combination in a singleimplementation. Conversely, various features that are described in thecontext of a single implementation can also be implemented in multipleimplementations separately or in any suitable sub-combination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multi-tasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure. Accordingly; otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A packet switch comprising: a first stage switchfabric receiving an electrical signal; a non-transitory mid-stage bufferreceiving and storing the electrical signal from the first stage switchfabric; and a second stage switch fabric receiving the electrical signalfrom the non-transitory mid-stage buffer, wherein each switch fabriccomprises: N layers of N×N arrayed waveguide gratings, each arrayedwaveguide grating having ingress ports and egress ports; a wavelengthtunable device in communication with a source ingress port of an arrayedwaveguide grating, the wavelength tunable device converting the receivedelectrical signal to an optical signal having a wavelength selected forrouting a packet from the source ingress port to a target egress port ofthe arrayed waveguide grating; and a photoreceiver receiving thepropagated optical signal from the target egress port and converting theoptical signal to the electrical signal; and a scheduler incommunication with the wavelength tunable devices, the schedulerconfigured to receive a notification of a received packet and issuerouting instructions to the wavelength tunable devices for routing thepacket from a source ingress port to a target egress port on each switchfabric.
 2. The packet switch of claim 1, further comprising an interfacereceiving an optical signal and converting the optical signal to theelectrical signal received by the first stage switch fabric.
 3. Thepacket switch of claim 2, wherein the interface comprises an opticaltransceiver.
 4. The packet switch of claim 2, further comprising anon-transitory input buffer receiving and storing the electrical signalfrom the interface, the non-transitory input buffer communicating theelectrical signal to the first stage switch fabric.
 5. The packet switchof claim 4, wherein the scheduler receives the notification from theinterface of a.
 6. The packet switch of claim 5, wherein the schedulerissues instructions in parallel to the first and second stage switchfabrics.
 7. The packet switch of claim 1, wherein the photoreceivercomprises a burst-mode photoreceiver.
 8. The packet switch of claim 1,wherein each switch stage fabric is connect in a Clos network.
 9. Thepacket switch of claim 1, wherein the wavelength tunable devicecomprises a tunable laser.
 10. The packet switch of claim 1, whereineach switch stage fabric comprises 100 layers of 100×100 arrayedwaveguide gratings having a per port speed of about 10G.
 11. A method ofpacket switching, the method comprising: receiving an electric signal;receiving a notification of a received packet on the electronic signal;determining routing instructions for the packet; converting the electricsignal to a first optical signal based on the routing instructions;propagating the first optical signal through a first stage switchfabric, the first stage switch fabric comprising N layers of N×N arrayedwaveguide gratings, each arrayed waveguide grating having ingress portsand egress ports, the first optical signal having a first wavelengththat results in routing the packet from a source ingress port to atarget egress port of an arrayed waveguide grating of the first stageswitch fabric; converting the first optical signal back to the electricsignal; after converting the first optical signal back to the electricsignal, storing the electrical signal in a non-transitory mid-stagebuffer; converting the electric signal to a second optical signal basedon the routing instructions; propagating the second optical signalthrough a second stage switch fabric, the second stage switch fabriccomprising N layers of N×N arrayed waveguide gratings, each arrayedwaveguide grating having ingress ports and egress ports, the secondoptical signal having a second wavelength that results in routing thepacket from a source ingress port to a target egress port of an arrayedwaveguide grating of the second stage switch fabric; and converting thesecond optical signal back to the electric signal.
 12. The method ofclaim 11, further comprising receiving an input optical signal andconverting the input optical signal to the electrical signal.
 13. Themethod of claim 12, further comprising storing the electrical signalwhile waiting for instructions for converting the electrical signal tothe first optical signal having the first wavelength.
 14. The method ofclaim 11, further comprising, after converting the first optical signalback to the electric signal, storing the electrical signal while waitingfor instructions for converting the electrical signal to the secondoptical signal having the second wavelength.
 15. The method of claim 11,further comprising receiving in parallel routing instructions forconverting the electrical signal to the first optical signal and routinginstructions for converting the electrical signal to the second opticalsignal.
 16. The method of claim 11, further comprising converting thesecond optical signal back to the electric signal using a burst-modephotoreceiver.
 17. The method of claim 11, further comprising connectingthe first and second stage switch fabrics in a Clos network.
 18. Themethod of claim 11, wherein each switch fabric comprises 100 layers of100×100 arrayed waveguide gratings having a per port speed of about 10G.